Like Qrystal, I left 'another' task manager because of their unintuitive 'check box to select', and came to Toodledo instead.
On a paper-based todo-list, you also check boxes to mark as complete after all...

I guess Gmail slanted everyone's thinking! To my eyes, in a general web form selecting means precisely that, to select something. In a task manager based web form, I fully agree with Toodledo's implementation of checkbox selection = task completion... it just makes sense. Heavy Gmail users may like RTM's keyboard shortcuts and method of selection / completion better.